"As for the Tony/Pepper storyline, we wanted to show what happens four movies later in the course of a relationship, because as Kevin Feige [producer] reminded me, Tony Stark is the only Super Hero who has cultivated and maintained a relationship with one woman over time that doesn’t go south entirely.
I start thinking in terms of what’s in it for my co-stars, and what is it that keeps this interesting for Gwyneth. We have addressed that because she has a pretty remarkable arc for a love-interest, female heroine in this film. So that’s probably one of the things that I am most excited about. Without giving it away, let’s just say she has a pretty incredible arc this time.”
Robert Downey Jr. on Tony and Pepper’s relationship